This invention relates, in general, to current source circuits and, in particular, to current source circuits that generate current with linear positive temperature dependence.
In some electronic circuits, it is desirable or necessary to have a current source that is regulated to compensate for changes that occur within the circuit due to temperature. An example of such a current source is called a proportional to absolute temperature (PTAT) current source that is used as a tail current in a differential bipolar amplifier, as illustrated in FIG. 1, to maintain a constant gain in the presence of temperature variations.
However, a PTAT current source may be inadequate in amplifiers that transfer power to a load, which is determined by the value of the bias current. The reason for this is that at cold temperatures, the PTAT drops to a lower value compared to its value at room temperature and, therefore, the power that is intended to be transferred will not develop across the load due to the diminished bias current at cold temperatures.
Conceivably, a gain chain, similar to that of a communications system, may have the last amplifier biased at a constant current, so as to deliver the proper power to a load, while the other gain components in the chain will compensate for the gain variations in the last amplifier and for the self-gain variations. In such a case, a current source having a slope that is different from that of a PTAT current source is required.
FIG. 2 depicts typical PTAT current slopes. Three values are provided to show that PTAT current changes linearly with respect to temperature and is always 0 at 0 K. However, this temperature dependency limits the application of PTAT currents. Thus, it is desirable to have a current source that provides the same current value at room temperature (i.e., examining line B), but has a slope other than the PTAT slope.
Other applications may be in circuits where an electrical parameter is a function of temperature and with a current that varies with an arbitrary slope, these electrical variations are canceled. A particular example would be a MOS transistor, biased in the linear region to manifest a resistance. This resistance is, in part, a function of temperature. By adjusting the gate voltage appropriately, the variations in resistance due to temperature can be cancelled. The gate voltage can be generated by first using a current source linearly dependent on temperature at the appropriate slope and then converting this current to a voltage (e.g., by impressing it on a resistor) and applying the voltage to the gate of the MOS device.
It is an objective of the present invention to provide a new and improved current source that is linearly dependent on temperature.
It is another objective of the present invention to provide a current source that has a predetermined current versus temperature slope.
An apparatus for generating a current linearly dependent on temperature, constructed in accordance with the present invention, includes a proportional to absolute temperature current source for generating a proportional to absolute temperature current and a constant current generation circuit responsive to the proportional to absolute temperature current for generating a constant current independent of temperature. An apparatus for generating a current linearly dependent on temperature, constructed in accordance with the present invention, also includes current combining means coupled to the proportional to absolute temperature current source and the constant current generation circuit for combining the proportional to absolute current and the constant current independent of temperature to generate a linearly temperature dependent current with a predetermined slope by one of reducing the proportional to absolute temperature current by the constant current independent of temperature and increasing the proportional to absolute temperature current by the constant current independent of temperature.
A method for generating a current linearly dependent on temperature, according to the present invention, includes the steps of generating a proportional to absolute temperature current and generating a constant current independent of temperature. The method further includes combining the proportional to absolute temperature current and the constant current independent of temperature to generate a linearly temperature dependant current with a predetermined slope. The linearly temperature dependent current is generated by one of reducing the proportional to absolute temperature current by the current independent of temperature and increasing the proportional to absolute temperature current by the constant current independent of temperature.
The details of the preferred embodiment of the present invention are set forth in the accompanying drawings and the description below. Once the details of the invention are known, other additional innovations and changes will become obvious to one skilled in the art.